Traditionally a spar acts as a reinforcing beam in a wind turbine blade. The spar is located between two shell parts, one defining a windward side shell part and the other one defining a leeward side shell part. The spar is located in the cavity between the two wind turbine shell parts and extends substantially throughout the shell cavity in order to increase the strength of the wind turbine blade.
In order to increase the strength of the spar and in order to limit the weight hereof, composite materials are often used for spars to be used in wind turbine blades, since such blades are exposed to varying loads with high peeks.
Traditionally, a spar is a tubular element being manufactured by the use of a male mould, e.g. by winding a suitable material around a mandrel or a similar core element.
When winding or by other means applying a material onto a mandrel or a core, the inner geometry of the final tubular element is defined by the geometry of the mandrel or core, thus allowing for a well-defined inner geometry. On the contrary, the outer geometry of the final tubular element is less well-defined as the effect of even small variations on the mandrel or core and/or small variation on the innermost layers of the winded material are increased with the number of windings.
Alternatively, a spar is sometimes made from two separately moulded elements which subsequently are joined in order to define a tubular element. In order to achieve a tubular element of the right size, a height adjustment element can be applied to assure that the final spar fits in the cavity between the two shell parts defining the wind turbine blade.